Re: [AGG] Subpixel problem - 90/180 deg. lines

[Stephan A. <sup...@gm...>]

Am 07.03.2011 15:58, schrieb Phil Endecott:
> Oyvind Idland wrote:
>> When a line segment is 90 or 180 degrees (has same x or y), the subpixel
>> logic makes the lines appear as 50% transparent and twice as fat as they
>> should be.
>
> Presumably that is the standard issue of whether 1-px-wide lines are
> better centred on whole coordinates or on 0.5 coordinates, i.e. are
> "pixels" points or unit-squares.

If you have a 5 by 5 pixel rendering buffer, you can reach each location 
of that buffer with the coordinates (0.0, 0.0) to (6.0, 6.0). AGG sort 
of concerns itself only with fill polygons, stroked lines don't exist as 
a concept at the rasterizer level (well, they do with the outline 
rasterizer, but that's a special optimization use-case). It would be 
weird if you had to refer to the top left most corner of the rendering 
buffer with (-0.5, -0.5), or fill the entire rendering buffer with a 
polygon covering (-0.5, -0.5) -> (5.5, 5.5), just so people don't 
stumble over the non-aligned lines issue every once in a while.

Best regards,
-Stephan